Katarina is just straight up badly designed, that's her problem
She's got 4 damaging abilities and they're all technically. Included with the most snowbally passive of all mobas' history. Luden's echo doesn't exactly help either.
There is literally no point in nerfing her, it would change nothing. The only way to fix this champ is to give her a massive rework, at the scale of Sion, but due to so many people maining her, that would be impossible. Just focus your balancing on other parts of the game, and let us play draft in the meantime.
And no i'm not saying she's op, because some appearantly thought so.
; Numbers aside the only mechanics change she got was to Ricochet, yet (at least to me) she plays very differently now.